The present invention relates generally to wireless sensor networks for system monitoring and, more particularly, to communication schemes between elements of a wireless sensor network.
In sensor monitoring networks employing wireless communications systems, two relevant design issues are data latency and data outage. That is, the time it takes for data to travel from a network of sensors to a readout device (data latency) and the loss of more than a maximum allowable period of data (data outage) are two critical requirements to the performance of a wireless sensor network. In order for a wireless sensor monitoring network to function properly, both data latency and data outage must be within acceptable limits.
To address the issues of reliability and transmission delay, the concept of a beacon controlled sensor network, or so-called time division multiple access (TDMA) approach, has previously been used. In TDMA, various devices use the same transmission frequency channel by dividing a signal into different timeslots within a transmission frame. Signals are transmitted in rapid succession, one after the other, each using its own timeslot, which allows multiple devices to share the same transmission medium (e.g. radio frequency channel) while using only part of its bandwidth.
The use of a TDMA network approach, however, presents a number of challenges. That is, as set forth above, data latency is of great importance in wireless sensor monitoring networks, and thus, a delay in transmission of a packet of data from sensors to a remote receiver to fit within a specific timeslot may not always be feasible, depending on the number of slots present in the frequency channel. Thus, data transmission may be delayed until a subsequent frame.
Additionally, the data outage specifications for a wireless sensor monitoring network may, for example, require a transmission success rate of 95% for signals transmitted from the sensor nodes to the gateway in order to meet the performance requirements. For signals transmitted over a single frequency, such a success rate can be unlikely, as the sensors and the receiver can be in close proximity of an inteferer that occupies the same or adjacent radio frequency spectrum. Signals transmitted on neighboring frequencies may still present significant power within the sensor network's operating frequency, preventing successful message transfer. For example, 802.11b, Bluetooth, Zigbee, 802.15.4 and legacy 802.11 transmitters within a facility will cause problems if these interferers transmit at the same time the wireless sensor monitoring network is transmitting. While previous monitoring networks have implemented the practice of retransmitting a data message multiple times to increase the transmission success rate, such retransmission may cause the data to exceed the delay requirement, greatly reducing the value of the data.
In the context of patient health monitoring, wireless sensor networks may be quite useful, as they may circumvent problems associated with cable clutter and device management. The wireless transfer of data from patient sensors to a gateway device or wireless network allows for the management of such devices to be eased and the burdens associated with cable management to be lessened. However, as described above, the use of wireless sensor networks for patient monitoring requires that communications issues often associated with wireless communication systems be adequately addressed. For patient health monitoring, the issues of data latency and data outage can be extremely problematic. That is, vital sign monitoring (e.g., monitoring of blood pressure, hemoglobin saturation, and features of the electrocardiogram (ECG)) is an important part of patient care since the general or particular health of the patient is determined, in part, through measurement and interpretation of key physiological indicators. Such physiological data, however, is only of use if it is transmitted in a timely and accurate manner. Transmission of such vital sign data must therefore be timely and be transmitted at a high rate of success in order for a wireless patient sensor network to be feasible.
Therefore, it would be desirable to design an apparatus and method that minimizes data latency and data outage. It would also be desirable to design a system and method that allows for timely and reliable data transmission when located in proximity to an inteferer that occupies the same or adjacent radio frequency spectrum. It would also be desirable to design a system which accomplishes this while exchanging a minimum of control messages that contain no data but consume power and frequency resources.